This invention relates to creating a gypsum fire barrier in cable trays or raceways. It does not relate to forming a sheathing or encasement around individual cables over their entire length as in the manufacture of power, instrument and communication cables and the like; but rather to where a plurality of such manufactured cables in usage placement are in a run, tray or raceway and discrete gypsum composition blocks are separately, individually, placed at interrupted intervals around a section of the cables to form a barrier against fire spread from one side to the other side of the block.
The possibility of fires in areas with high concentrations of instrument and communication wires and power transmission cables has become of great concern to power companies, automated industrial plants and government regulatory bodies. Power, instrument and communication cables constructed with combustible coverings or sheathings can provide a pathway by which fire might spread. Physical discrete barriers (e.g. floors, walls, and ceilings) can serve to contain a cable fire to a given room. However, even at such points of barrier the necessary holes for the cables to pass through walls, floors and ceilings can still provide a means for transmission of the fire to adjoining rooms. Cable fires within a room, further, need to be arrested by the construction of a properly designed "fire break" placed periodically along the cable run.
Cable manufacturers are aware of this problem and have directed efforts towards developing cable insulation characterized by lower flame spread and reduced fuel load. There are also prior attempts to provide insulative coatings or sheathings in the cable manufacturing process. For example, U.S. Pat. No. 2,077,282 discloses constructing a cable with an inner insulated sheath between the cable core and an outer sheath. The insulated sheath, to be selected from a material or mixture of boric acid, calcined gypsum or plaster of Paris, borax, magnesium carbonate, basic magnesium carbonate, ammonium carbonate, aluminum ammonium sulfate, or the carbonates or bicarbonates of the alkali or alkaline earth metals is said to be selected as avoiding explosive gases when an arc in the cable occurs by decomposition of the material by the heat of the arc. That patent further indicates that such materials may be substituted for in part with an inert material such as asbestos. In addition, U.S. Pat. No. 2,207,579 discloses a fire resistant cable in which a layer of spun glass threads bound by a layer of molten glass insulation is applied as the insulating layer between the inner conductor and the outer sheathing, in this case an outer metal casing. U.S. Pat. No. 3,324,232 discloses a transmission cable having an inner sheathing of solid boron nitride within an outer lead sheathing. Only slightly related U.S. Pat. 3,531,678 discloses a heater wire in which the heating filament is surrounded by a boron nitride coating, stating that such coating without an outer sheathing is electrically insulating but thermally conductive. Thus such materials generally depend upon an outer sheathing of electrically conductive, non-insulative material, to provide for an electrical ground. However, such are also known for their attendant costs; and the brittleness of the inner sheathing between the core and the outer metallic sheathing considerably limits flexability of such cables.
Of course, other materials have been attempted without utilizing the outer metallic sheathing and have generally centered around rendering organic sheathing materials more resistant to fire by the inclusion of fire retardant additives. It does not, however, appear feasible to produce an organic cable insulation which is totally noncombustible or to replace all of the existing highly combustible cable that is already in use.
Divergent from that area of usage it is also known from U.S. Pat. No. 3,393,116 to form a plaster composition for use in thin coat plaster coatings over all surfaces that utilize radiant heating systems. In such systems an electrically resistant element is secured over a gypsum board plaster base and embedded beneath a thin veneer coat plaster surface, with the veneer plaster coating containing small amounts of glycerol and boric acid intended to form a surface adapted to be subjected to temperatures as high as 150.degree. F. without deleterious cracking and strength deterioration of the 1/16-1/8 inch thick veneer layer. In other nonrelated attempts at fire resistant compositions that include gypsum, U.S. Pat. No. 3,885,980 discloses containers, such as burial vaults and safes having an inner liner of a material such as glass fiber mat and a gypsum composition containing as essential included ingredients a magnesium sulfate heptahydrate. That patent notes that borax upon trial proved to be deleterious in its effects upon the setting of plaster of Paris and in disastrously fluxing at high temperatures.
Returning again now to the proposal of a suitable "fire break"/"fire stop" material for usage in cable trays; it is recognized that power, communication and instrumentation cables sheathed with combustible insulation and packed into cable trays or along raceways provide a fuel source for fires. It is not always feasible to either produce a cable insulation which is totally noncombustible nor to replace all of the existing highly combustible cable that is in place in such trays. Thus it would be ideal to place barriers at intervals along such trays or raceways. A candidate for such a fire barrier material, in addition to preventing the spread of flame, must meet several additional functional requirements. Recent electrical code changes have decreased the amount of cable spacing clearance both within trays and between tiers of trays, the trend being to now allow just sufficient space about the trays to permit adequate access for installation and maintainance of the cables. Further, revised electrical codes now permit installation of conductor cables rated 600 volts or less in the same tray; but another section prohibits the mixing of cables rated over 600 volts with cables rated 600 volts or less in the same tray unless the cables are (1) separated by solid, noncombustible, fixed barriers or are (2) type MC (metal-clad) cable.
Prior attempts to formulate fire barriers in cable electrical trays and raceways have heretofore concentrated upon either organic silicone formulations, which are quite expensive; or urethane foam formulations to which a fire retardant chemical has been added, which are not entirely satisfactory in their performance characteristics. Of course, one might think of turning to an inorganic cementitious material for such use but previous attempts have been highly unsatisfactory since most inorganic cementitious materials either shrink excessively upon drying or are not sufficiently expansive after set to remain crack-free or void-free in usage. In addition such formulations may be sufficiently fluid to surround the cables and fill the voids and interstices below and between cables within a packed tray, but are then so fluid that they flow along the tray and away from the area to be filled. Inorganic cementitious materials such as Portland cement suffer an important prohibitive defect in that the fire barrier composition must be sufficiently frangible and of limited strength that a portion of the fire break/fire stop may be readily torn out for the replacement, repair, or addition of cables to the tray or raceway. Quite frequently at some time after an initial raceway has been set up it is mandatory to either remove an existing cable or add additional cables. Ordinary Portland cement develops dry cast compressive strengths in excess of 6,000 pounds per square inch. Ordinary gypsum casting plasters develop about 2,000 p.s.i. Inorganic cementitious materials such as Portland cement have caused the complete destruction of the whole raceway section in attempts to remove a portion of the fire stop for replacement of one or more cables in that raceway. Also the weight of fire barriers with such materials has caused the buckling and collapse of the cable trays.
Thus a summary of functional requirements for a "fire barrier" material is that the material should be totally noncombustible; should provide a substantial heat sink; should in application completely surround the cables and fill all the voids and interstices; be sufficiently viscous to minimize lateral flow along the tray or raceway; be easily removed to permit adding or removing calbes in the tray; be relatively homogeneous and crack free; be low density to not require additional support for cable assemblies; not shrink on drying and setting; and lastly not have any deleterious effect on any of the cables, ancillary insulation, sheathing or jacket of the cables or on the cable tray itself.